In fairness, probably very few of them are what I would describe as evil by design (Mr Cheney, I'm looking at you here). The problem is, as Iain Banks noted in Complicity, if you combine thundering incompetence with the desire for power  you're bound to wind up with massive and costly errors, for which you are directly responsible, and wielding power you know (or should know) you are not capable of using sensibly becomes evil to all intents and purposes.
Having spent so much time yesterday arguing that you can't make these generalisations and not offer up a shred of evidence, here's John Boehner, a man who is only not Speaker of the House because the Democrats are in the majority right now.
George, the idea that carbon dioxide is a carcinogen that is harmful to our environment is almost comical. Every time we exhale, we exhale carbon dioxide. Every cow in the world, you know, when they do what they do, you've got more carbon dioxide.Is he deliberately evil, or so incompetent and ignorant that he might as well be? "Carcinogen" is a word used to describe a substance or phenomenon that has been linked with cancer. If anyone has made the argument that CO2 is a carcinogen, it's news to me. More obviously, though, the man who may be the next Speaker of the House believes that something can't be bad if cows fart it. That, ladies and gentlemen, is his argument. It's pretty irritating when people deliberately ignore subtleties of an argument for the sake of a soundbite, but in this case the "subtlety" is that thanks to us there are a billion fucking cows in the world. Which produce methane, by the way, which is much more of a concern than CO2, though at this point I'm amazed he even managed to correctly identify cows as being part of the discussion.
Incompetence + Power = Evil. There's partisan politics, and then there's just knowing who the enemy is.
 Note my cunning sleight-of-hand here, referring to the party as a single unit to prevent me from having to deal with them as individuals. I'm quite sure there are plenty of Republicans who are neither evil nor stupid. As several people more smart than I have have noted, though, the problem with the GOP elders is that stupidity is not seen as an impediment for progress, whereas intelligence is viewed with suspicion.
 To be strictly accurate, in that book the observation was made regarding a doctor killing a patient through incompetence, though the more general point was driven home too.